This invention relates to improvements in photomasks, especially hard masks, for use in photoetching.
Conventionally, the photoetching technique is employed in the fabrication of semiconductor integrated-circuit devices. In patterning the photoresist applied to a semiconductor substrate, a photomask is brought into contact with the photoresist and then exposed to ultraviolet light. Although various methods of exposure such as contact exposure and projection exposure are available, the contact exposure method is commonly employed from the viewpoint of resolving power and cost. The projection exposure method is only employed for special purposes. The prior art photomasks include emulsion masks provided with a mask layer having a thickness of several microns and what is called hard masks provided with a mask layer of metal oxide having a thickness of at most 3000 A. For purposes of minute processing to a line width of 6.mu. or less, hard masks are largely employed from the viewpoint of resolving power and mask life.
Meanwhile, the photoresists commercially available for the fabrication of semiconductor integrated-circuit devices evolve nitrogen gas on exposure to ultraviolet light, whether they may be of the positive or negative type. According to the method of exposure in which a photomask is brought into close contact with a wafer under a pressure of, for example, 100 g/cm.sup.2, the nitrogen gas so evolved accumulates between the photomask and the wafer, so that part of the photomask may go out of contact with the wafer and thereby cause a reduction in resolving power. In the case of a photomask of the emulsion type having a mask layer as thick as several microns, the nitrogen gas escapes to the outside through the grooves formed in the mask surface. Even if the nitrogen gas accummulates in the grooves, the pressure does not rise to such a degree as to degrade the quality of contact. Consequently, any reduction in resolving power due to poor contact does not occur. In the case of a hard mask, however, the grooves formed in the mask surface is so shallow that the nitrogen gas scarcely escapes therethrough and causes a reduction in resolving power due to poor contact. Since the elimination of such poor contact is essential for the purpose of increasing the yield of products, there has been a strong demand for proper countermeasures.